Chocolate bloom

X-ray study may aid in making better chocolate

One of the major problems in the confectionery industry is chocolate ‘blooming’, the formation of white defects on the chocolate surface due to fat crystals. A new study looks at a better understanding of blooming and ways to mitigate it.

The study, recently published online in the journal ACS Applied Materials & Interfaces outlines how the process of blooming is often related to the migration of lipids to the surface followed by subsequent recrystallization.

“This can happen when liquid chocolate cools down in an uncontrolled manner and unstable crystals form, for example. But even at room temperature, a quarter of the lipids contained in chocolate are already in a liquid state. Liquid fillings or ingredients such as nougat can accelerate the formation of fat bloom,” said Dr Svenja Reinke from the Institute of Solids Process Engineering and Particle Technology in Hamburg, Germany, lead author on the study.

The coating is edible, but it changes the appearance and texture of the product. The longer chocolate is stored and the higher the storage temperature, the more likely fat blooming is observed. This is why the white colouration of the product surface is often taken to be a sign of old or spoiled chocolate.

In the new study, Dr Reinke and her colleagues wanted to find out what factors were contributing to the formation of fat blooms. Using DESY’s X-ray source PETRA III, the researchers investigated the microscopic structural changes that occur when chocolate blooms.

They found that the lipids that are responsible move through pores and cracks in the chocolate. Along the way, they soften and dissolve solid cocoa butter into a liquid form.

“The technology used to examine the samples shows us both the fat crystals and the pores inside the product, down to a scale of a few nanometers,” said study senior author Prof Stefan Heinrich of the Institute of Solids Process Engineering and Particle Technology.

“Our results showed that the chocolate powders get wet by the oil during the migration process and that the oil is migrating into the pores within seconds. Subsequently, cocoa butter is dissolved by the oil, and thus, its characteristic crystalline structure is lost.”

The scientists said that reducing the porosity of the chocolate during manufacture could help minimise blooms.

“Another approach is to limit the amount of fat that is present in a liquid form by storing the product in cool, but not too cold, conditions. 18 degrees Celsius is ideal,” Dr Reinke said.

“Chocolate is very sensitive to fluctuations in temperature. Just a few degrees make a big difference. At 5 degrees, basically, all cocoa butter is solid; and above about 36 degrees everything is liquid. On top of this, the type of crystals in the chocolate plays an important role.”

“Cocoa butter crystallizes in six different crystal forms. The amount of fluid also depends on the form of the crystals. Manufacturers can also limit fat bloom by controlling crystallization.”

Journal Reference:

Svenja K. Reinke et al.

Tracking Structural Changes in Lipid-based Multicomponent Food Materials due to Oil Migration by Microfocus Small-Angle X-ray Scattering.

ACS Appl. Mater. Interfaces, published online April 20, 2015; doi: 10.1021/acsami.5b02092